EERI Seminar on Next Generation Attenuation Models New “Risk-Targeted” Seismic Maps Introduced...

EERI Seminar on Next Generation Attenuation Models

New “Risk-Targeted” Seismic Maps Introduced into USA Building Codes

Nicolas Luco, Research Structural Engineer

U.S. Geological Survey

Golden, Colorado

Indian Institute of Science (IISc) Department of Civil Engineering Seminar


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120° 110° 100°

45°

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35°

30°

REFERENCES

Building Seismic Safety Council, 2009, NEHRP Recommended Seismic Provisions for New Buildings and Other Structures: FEMA P-750/2009 Edition, Federal Emergency Management Agency, Washington, DC.Huang, Yin-Nan, Whittaker, A.S., and Luco, Nicolas, 2008, Maximum spectral demands in the near-fault region, Earthquake Spectra, Volume 24, Issue 1, pp. 319-341.Luco, Nicolas, Ellingwood, B.R., Hamburger, R.O., Hooper, J.D., Kimball, J.K., and Kircher, C.A., 2007, Risk-Targeted versus Current Seismic Design Maps for the Conterminous United States, Structural Engineers Association of California 2007 Convention Proceedings, pp. 163-175.Petersen, M.D., Frankel, A.D., Harmsen, S.C., Mueller, C.S., Haller, K.M., Wheeler, R.L., Wesson, R.L., Zeng, Yuehua, Boyd, O.S., Perkins, D.M., Luco, Nicolas, Field, E.H., Wills, C.J., and Rukstales, K.S., 2008, Documentation for the 2008 Update of the United States National Seismic Hazard Maps: U.S. Geological Survey Open-File Report 2008-1128, 61 p.

DISCUSSION

Maps prepared by United States Geological Survey (USGS) incollaboration with the Federal Emergency Management Agency(FEMA)-funded Building Seismic Safety Council (BSSC) andthe American Society of Civil Engineers (ASCE). The basis isexplained in commentaries prepared by BSSC and ASCE and inthe references. Ground motion values contoured on these maps incorporate: • a target risk of structural collapse equal to 1% in 50 years based upon a generic structural fragility • a factor of 1.1 to adjust from a geometric mean to the maximum response regardless of direction • deterministic upper limits imposed near large, active faults, which are taken as 1.8 times the estimated median response to the characteristic earthquake for the fault (1.8 is used to represent the 84th percentile response), but not less than 150% g. As such, the values are different from those on the uniform-hazard 2008 USGS National Seismic Hazard Maps posted at:http://earthquake.usgs.gov/hazmaps. Larger, more detailed versions of these maps are not providedbecause it is recommended that the corresponding USGS webtool (http://earthquake.usgs.gov/designmaps orhttp://content.seinstitute.org) be used to determine the mappedvalue for a specified location.

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50° 90° 80° 70°

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100 0 100 200 300 400 500 Miles

100 0 100 200 300 400 500 Kilometers

Explanation

Contour intervals, %g

3002001501251009080706050403530252015105

Areas with a constant spectral response acceleration of 150% g

10101010

E16.9

Point value of spectral response acceleration expressed as a percent of gravity

Contours of spectral response acceleration expressed as a percent of gravity. Hachures point in direction of decreasing values

2012 “International” Building Code

FIGURE 1613.3.1 RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER) GROUND MOTION …


“New Risk-Targeted Seismic Maps Introduced into USA Building Codes,” N. Luco, USGS March 6, 2013


Development of MCER Maps



10

7-10


http://earthquake.usgs.gov/designmaps/usapp/




Outline of Presentation

• Past/present design approaches for seismic safety

• Quantification of seismic risk/performance

• Recent/future risk-based seismic design approaches

• Implications for designing against hazards, in general

• Outstanding issues




Past Seismic Design Approaches

• Past and most present design approaches for seismic safety are prescriptive, based on following rules rather than explicitly quantifying performance/risk.

e.g., “If at least seven ground motions [(time series)] are analyzed, the design member forces … are permitted to be taken … as the average of the … values determined from the [building response] analyses …” – ASCE 7 Standard

• For earthquake loads/demands, past (and most present) approaches typically design against …




Deterministic Earthquake Scenarios



e.g., USGS “ShakeMaps”


Probabilistic Seismic Hazard Maps



e.g., ground motion intensity with a uniform 1/2500 annual probability of being exceeded

EERI Seminar on Next Generation Attenuation ModelsIndian Institute of Science (IISc) Department of Civil Engineering Seminar


Past Seismic Design Approaches

Although seismic safety was implied by these past design approaches, …

… the seismic risk for the resulting structures …

(e.g., annual probability of seismically-induced failure)

… was not explicitly quantified in the design process, or in the development of the design process by regulators.


Meanwhile, for decades now (e.g., ATC 3-06, 1978) …

Seismic Risk Quantification



EarthquakeScientists

EarthquakeEngineers

SeismicRisk

Probabilistic SeismicFragility Curves

Probabilistic SeismicHazard Curves


Probabilistic Seismic Hazard Curves

Results of Probabilistic Seismic Hazard Analysis (PSHA), pioneered by the late Prof. C. Allin Cornell in 1968.

Notes:

Hazard curves are interpolated to derive aforementioned “hazard maps”.

Shapes of hazard curves can vary significantly with location.



Ground Motion Intensity

Ann

ual P

rob.

of

Exc

eeda

nce


10-2

10-1

100

101

0

0.1

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0.8

0.9

1

Ground Motion Intensity, IM = z

Pro

b [

Col

laps

e |

IM =

z ]

Design IM = 1.38gDesign IM = 0.96g

Based on the performance of similar structures in past earthquakes, and/or numerical simulations.

Probabilistic Seismic Fragility Curves

Notes:

Fragility curve depends on the ground motion intensity for which the structure was designed (“Design IM”).

It also depends on the prescriptive requirements that are used to design the structure (e.g., those for buildings vs. nuclear power plants).




“Risk Integral”

Combination of such hazard and fragility curves is an application of the total probability theorem …



𝑷ሾ𝑪𝒐𝒍𝒍𝒂𝒑𝒔𝒆ሿ= න d𝑷ሾ𝑪𝒐𝒍𝒍𝒂𝒑𝒔𝒆ȁ 𝑰𝑴= 𝒛ሿ d𝒛 𝑷ሾ𝑰𝑴> 𝑧ሿ d𝒛∞

𝟎

HazardRisk Fragility


“Risk Integral” (Example)



Haz

ard

Ris

kF

rag

ilit

y

Ground Motion Intensity, IM=z

P [

Col

laps

e | I

M=

z ]

P [

IM >

z ]

Inte

gran

d



Risk-Based Design Approaches

• Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motions for designing new buildings and other structures, 2012 International Building Code

• A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion, U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.208, 2007

• Development of Next-Generation Performance-Based Seismic Design Procedures for New and Existing Buildings, Applied Technology Council (ATC) Project #58, funded by U.S. Federal Emergency Management Agency (FEMA), 2012




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6.813.9

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80

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150

150200

125

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150150

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120° 110° 100°

45°

40°

35°

30°

REFERENCES

Building Seismic Safety Council, 2009, NEHRP Recommended Seismic Provisions for New Buildings and Other Structures: FEMA P-750/2009 Edition, Federal Emergency Management Agency, Washington, DC.Huang, Yin-Nan, Whittaker, A.S., and Luco, Nicolas, 2008, Maximum spectral demands in the near-fault region, Earthquake Spectra, Volume 24, Issue 1, pp. 319-341.Luco, Nicolas, Ellingwood, B.R., Hamburger, R.O., Hooper, J.D., Kimball, J.K., and Kircher, C.A., 2007, Risk-Targeted versus Current Seismic Design Maps for the Conterminous United States, Structural Engineers Association of California 2007 Convention Proceedings, pp. 163-175.Petersen, M.D., Frankel, A.D., Harmsen, S.C., Mueller, C.S., Haller, K.M., Wheeler, R.L., Wesson, R.L., Zeng, Yuehua, Boyd, O.S., Perkins, D.M., Luco, Nicolas, Field, E.H., Wills, C.J., and Rukstales, K.S., 2008, Documentation for the 2008 Update of the United States National Seismic Hazard Maps: U.S. Geological Survey Open-File Report 2008-1128, 61 p.

DISCUSSION

Maps prepared by United States Geological Survey (USGS) incollaboration with the Federal Emergency Management Agency(FEMA)-funded Building Seismic Safety Council (BSSC) andthe American Society of Civil Engineers (ASCE). The basis isexplained in commentaries prepared by BSSC and ASCE and inthe references. Ground motion values contoured on these maps incorporate: • a target risk of structural collapse equal to 1% in 50 years based upon a generic structural fragility • a factor of 1.1 to adjust from a geometric mean to the maximum response regardless of direction • deterministic upper limits imposed near large, active faults, which are taken as 1.8 times the estimated median response to the characteristic earthquake for the fault (1.8 is used to represent the 84th percentile response), but not less than 150% g. As such, the values are different from those on the uniform-hazard 2008 USGS National Seismic Hazard Maps posted at:http://earthquake.usgs.gov/hazmaps. Larger, more detailed versions of these maps are not providedbecause it is recommended that the corresponding USGS webtool (http://earthquake.usgs.gov/designmaps orhttp://content.seinstitute.org) be used to determine the mappedvalue for a specified location.

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35

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50° 90° 80° 70°

45°

40°

35°

30°

25°

100 0 100 200 300 400 500 Miles

100 0 100 200 300 400 500 Kilometers

Explanation

Contour intervals, %g

3002001501251009080706050403530252015105

Areas with a constant spectral response acceleration of 150% g

10101010

E16.9

Point value of spectral response acceleration expressed as a percent of gravity

Contours of spectral response acceleration expressed as a percent of gravity. Hachures point in direction of decreasing values

2012 International Building Code

FIGURE 1613.3.1 RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER) GROUND MOTION …




“Risk Integral” (Example)



Haz

ard

Ris

kF

rag

ilit

y


P [

Col

laps

e | I

M=

z ]

P [

IM >

z ]

Inte

gran

d



2

2

Risk-Targeted Ground Motions

H

azar

dR

isk

Fra

gil

ity



DD


P [

Col

laps

e | I

M=

z ]

P [

IM >

z ]

Inte

gran

d




H

azar

dR

isk

Fra

gil

ity



DD


P [

Col

laps

e | I

M=

z ]

P [

IM >

z ]

Inte

gran

d




“Guess” RTGMi

P[Collapse] in 50yrs= 1%?

Generate fragility curve as a function of RTGMi

Integrate fragility & hazard curves to calculate risk

RTGM CalculatedFor

a g

iven

loca

tion

…

Yes

No




Risk-Targeted GM Maps

Note: RTGM maps are coupled with deterministic GM maps to produce the MCER GM maps in the 2012 IBC




Risk Coefficients (CR’s)

• Conventional uniform-hazard (2500-yr) GMs interpolated from hazard curves

• CR’s =

• e.g., SFBA Location MMA Location

Risk-Targeted GM 1.38g 0.96g

Uniform-Hazard GM 1.29g 1.18g

Risk Coefficient (CR) 1.07 0.82

Risk-Targeted GMs

Uniform-Hazard GMs




Risk Coefficient (CR) Maps

125 W 120

W 115 W 110

W 105 W 100

W 95

W 90

W 85

W 80 W 75

W 70

W 65 W

25 N

30 N

35 N

40 N

45 N

50 N

Risk-Targeted (1% in 50yrs, = 0.8) Uniform-Hazard (2% in 50yrs) ; 0.2sec SA

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6





125 W 120

W 115 W 110

W 105 W 100

W 95

W 90

W 85

W 80 W 75

W 70

W 65 W

25 N

30 N

35 N

40 N

45 N

50 N


0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

< 0.85





125 W 120

W 115 W 110

W 105 W 100

W 95

W 90

W 85

W 80 W 75

W 70

W 65 W

25 N

30 N

35 N

40 N

45 N

50 N


0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

> 1.15




New MCER GMs

Preparation of New Design Maps

Deterministic GMsProbabilistic GMs(Risk-Targeted)

─ In ASCE 7-10─ In 2009 NEHRP

Risk CoefficientsUniform-Hazard GMs

X

To relate back to conventional uniform-hazard (2500-yr) GMs …



Google “EERI NGA Seminar Presentations” for Videoor email [email protected] for just PowerPoint

mailto:[email protected]


Summary

• Previous uniform-hazard (2%-in-50yr) probabilistic ground motions …

– Resulted in spatially-variable collapse risk, due to variations in hazard curve shapes

– Considered only a single selected point (2%-in-50yr) on hazard curves

– Were similar in value in Memphis Metro Area and San Francisco Bay Area

• New risk-targeted probabilistic ground motions address these issues




Summary (continued)• New risk-targeted probabilistic ground motions

(RTGMs) …

– Explicitly & uniformly target 1% probability of collapse in a building’s lifetime, ~50 years

– Consider all points on & spatial variations in shapes of hazard curves

– Require a generic fragility that depends on RTGM & effectively considers shapes of hazard curves

– Changes uniform-hazard (2%-in-50yr) ground motions by factor of 0.85-1.15 generally, but as low as 0.70 near New Madrid and Charleston




U.S. NRC RG 1.208 (2007)

“Target Risk” = 10-5 annual probability of Onset of Significant Inelastic Deformation (conservative w.r.t. structural failure)



Ground Motion Intensity, IM

Ann

ual P

rob.

of

Exc

eeda

nce Design IM presented as

…

where Design Factor is

and

}6.0,0.1max{DF 8.0RA

)10(/)10( 45 IMIMAR

DF)10(Design 4 IMIM


ATC-58 Project (funded by FEMA)

• In addition to building collapse, these new design procedures target annual probabilities of earthquake-induced …

– casualties (Deaths), – repair costs (Dollars), and – loss of use (Downtime)

… via generalizations of the risk integral.

• Furthermore, risk quantification is “high-resolution,” i.e., …

… fragility curve depends on details of individual elements of the building, not just the design ground motion intensity




Risk-Based Design Approaches



Seismic Risk Quantificationfor Existing Structures

Seismic Designof New Structures

Risk-Based Design


Implications

• Tolerable seismic risk has now been specified quantitatively (vs. qualitative “safe”), which in turn impacts, e.g., …

– evaluation and retrofit of existing structures,

– design for other hazards (e.g., wind, rock falls), and

– prescriptive design procedures (e.g., selection and modification of ground motion time series for structural response simulations).

• Risk-based approaches might eventually allay the need for prescriptive design procedures altogether.




Outstanding Issues

• Acceptable/tolerable risk level?

– once in terms of deaths/dollars/downtime, might be borrowed from other hazards?

– for individual structures and aggregation/region?

– must it be geographically uniform?

• Validation of risk modeling/quantification (vs. reality)

– hazard and fragility components/curves

– data very scarce at large ground motion intensities



EERI Seminar on Next Generation Attenuation Models New “Risk-Targeted” Seismic Maps Introduced...

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